Many basic cell functions that involve changes of cell shape and cell motion, such as during organism development or cancer cell metastasis, rely crucially on the properties of the actin cytoskeleton. Actin filaments in cells undergo constant assembly by polymerization and disassembly by severing and depolymerization. Large-scale actin filament network and bundle structures can have lifetimes much longer than those of individual filaments within them, as in lamellipodial protrusions where one third of all newly-formed actin filaments disassemble within 10 seconds, turning over before retrograde flow carries them through the lamellipodium. The short severed and depolymerizing actin filaments may diffuse and rebind to nearby filaments, a process that may determine the local network structure. In this project mathematical and computational models will be used together with experiments to study distributed actin turnover. Single Molecule Speckle (SiMS) microscopy allows dilute labeling of single proteins: when the fluorescent protein binds to the actin network t appears on an image as a speckle; when it is diffusing in the cytoplasm or on the membrane, it appears as a diffuse cloud. Thus SiMS provides a detailed view of reaction and diffusion in cells, which can be used in quantitative mathematical models to estimate concentration profiles, flows and actin network structure. Mathematical and computational models and SiMS will be combined to study kinetics of local remodeling in the lamellipodia and other cellular compartments such as adherens junctions. To study the molecular mechanisms that drive exploratory These remodeling dynamics are distributed in space. fluctuations of the actin cytoskeleton, lamellipodial protrusion and retraction will be used as a model system maintained in a steady state characterized by fluctuations in actin polymerization. Models will be developed to study the molecular mechanisms of positive feedbacks that amplify small fluctuations or perturbations as well the restorative mechanisms that maintain homeostasis and the role of slowly diffusible actin complexes in local regulation. To study the effect of local turnover on actn filament network structure, 3D models of remodeling networks will be tested against electron microscope tomography experiments. Finally, SiMS will be used together with mechanical perturbations of cells adhered to deformable substrates to model force transmission through dynamic actin networks as well as the effect of internal and external forces on actin turnover and network structure.